250 THE PHYSIOLOGY OF THE 



and teat. The teat has a coating of muscular substance 

 resembling the dartos in its nature ; while the main sub- 

 stance of the bag is made up of vessels for supplying blood, 

 and carrying it away again ; or of tubes or ducts for 

 carrying off the secretion. The ducts are lined by mucous 

 membrane reflected inward, and continuous with the integu- 

 ment ; the mammary gland being essentially a conglomerate 

 one. The milk is secreted into the minute glands, which 

 unite and unite again, till by an uncertain number of 

 branches they empty into ampulae, or dilatations ; whence the 

 milk is carried away by other ducts which communicate with 

 the tubuli lactiferi, which terminate at the point of the teat, 

 but whose number appears not to be fixed in the mare. 



The arteries are derived from the epigastric and the 

 femoral. The veins principally empty into the vena saphena 

 major and femoral trunks. The nerves are given off from 

 various adjacent sources. 



DESCRIPTION OF PLATE IX. 



THE ABDOMEN LAID OPEN. 



An incision has been made through the linea alba and the 

 w^alls of the cavity reflected back ; thereby affording a full 

 view of the intestines. Of course this proceeding has slightly 

 changed their exact relative situations ; but they are still 

 sufficiently true to enable the reader to form a tolerably 

 accurate idea of their positions when undisturbed. 



A, The coecum, having its blind extremity pointing down- 

 ward ; B, the colon originating and terminating in the right 

 hypochondriac region ; C, the rectum, showing one of its 

 two muscular bands, and the cell-like structure of this in- 

 testine also being prominently exhibited ; D, the small intes- 

 tines occupying the upper and posterior portion of the 

 cavity. Only a few of these last come into this view. 



SECTION XV. 



THE PHYSIOLOGY OF THE REPRODUCTIVE SYSTEM GENERALLY. 



That the great work of propagation might not be left to 

 chance, all animals are irresistibly impelled to it by the 



